Method for operating a chiller

ABSTRACT

A method of operating a chiller having a closed refrigerant loop including a compressor, a condenser and an evaporator. The refrigerant used in the loop defining a pressure-enthalpy curve representative of different phases (vapor, liquid and vapor, and liquid) of the refrigerant at different combinations of pressure and enthalpy. The loop defining a process cycle (compression, condensation, expansion, and evaporation) of the refrigerant during operation of the loop relative to the pressure-enthalpy curve of the refrigerant. The method including continuously operating the compressor when a segment of the process cycle corresponds to the refrigerant being in the liquid phase.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from and the benefit of PCT PatentApplication No. PCT/US2015/016734, entitled “METHOD FOR OPERATING ACHILLER,” filed Feb. 20, 2015, which is herein incorporated by referencein its entirety, and which claims priority to and benefit of U.S.Provisional Application Ser. No. 61/980,088, entitled “METHOD FOROPERATING A CHILLER,” filed Apr. 16, 2014, which is hereby incorporatedby reference in its entirety.

BACKGROUND

The application relates generally to refrigeration, air conditioning andchilled liquid systems. The application relates more specifically tomethods of operating refrigeration, air conditioning and chilled liquidsystems.

It has been recognized that under certain environmental conditions andreduced system cooling demand conditions, chilled liquid systems using acentrifugal compressor can be operated at a fraction of the cost whencompared to the cost during normal operation, sometimes referred to as“free cooling”. The 2008 ASHRAE Handbook—HVAC Systems and Equipment(page 42.12) provides as follows:

Cooling without operating the compressor of a centrifugal liquid chilleris called free cooling. When a supply of condenser water is available ata temperature below the needed chilled-water temperature, some chillerscan operate as a thermal siphon. Low-temperature condenser watercondenses refrigerant, which is either drained by gravity or pumped intothe evaporator. Higher-temperature chilled water causes the refrigerantto evaporate, and vapor flows back to the condenser because of thepressure difference between the evaporator and the condenser.

In other words, when the entering condenser water temperature is lessthan the exiting water temperature from the evaporator of a liquidcentrifugal liquid chiller, and when the demand for cooling issufficiently low such that the exiting evaporator water temperaturesatisfies the demand for cooling, the centrifugal compressor is shutoff, resulting in substantially energy savings.

Unfortunately, such environmental conditions in numerous parts of theworld relatively rarely occur, or may be brief in duration. Occurringyet more rarely is the combination of the advantageous environmentalconditions that simultaneously produce sufficient cooling output tosatisfy the demand for cooling, in order to permit shut-down of thecentrifugal compressor.

Thus, there is a need for a method of operating a chiller thatsignificantly increases the range of environmental conditions (e.g.,increases the range of temperatures of entering condenser watertemperatures and exiting evaporator water temperatures as well as therange of differences therebetween) to achieve energy savings duringchiller operation. There is a further need for a method of operating achiller in the above-referenced range of environmental conditions thatincreases chiller load capacity while simultaneously achieving suchenergy savings.

SUMMARY

The present invention relates to a method of operating a chiller havinga compressor including comparing the temperature of a liquid entering acondenser (“for thermal communication with refrigerant in thecondenser”) with a temperature of a liquid exiting an evaporator (forthermal communication with refrigerant in the evaporator). The methodfurther includes continuously operating the compressor at least inresponse to each temperature range: the liquid evaporator exitingtemperature being greater than the liquid condenser entering temperatureby a predetermined amount; the liquid evaporator exiting temperaturebeing substantially equal to the liquid condenser entering temperature;the liquid evaporator exiting temperature being less than the liquidcondenser entering temperature by a predetermined amount.

The present invention further relates to a method of operating a chillerhaving a closed refrigerant loop including a compressor, a condenser andan evaporator, the refrigerant used in the loop defining apressure-enthalpy curve representative of different phases (vapor,liquid and vapor, and liquid) of the refrigerant at differentcombinations of pressure and enthalpy, the loop defining a process cycle(compression, condensation, expansion, and evaporation) of therefrigerant during operation of the loop relative to thepressure-enthalpy curve of the refrigerant. The method includingcontinuously operating the compressor when a segment of the processcycle corresponds to the refrigerant being in the liquid phase.

The present invention yet further relates to a method of operating achiller having a centrifugal compressor including comparing thetemperature of a liquid entering a condenser (“for thermal communicationwith refrigerant in the condenser”) with a temperature of a liquidexiting an evaporator (for thermal communication with refrigerant in theevaporator). The method further includes continuously operating thecompressor using a VSD for controlling a rotational speed of acompressor motor, the compressor utilizing magnetic bearings, at leastin response to each temperature range: the liquid evaporator exitingtemperature being greater than the liquid condenser entering temperatureby a predetermined amount; the liquid evaporator exiting temperaturebeing substantially equal to the liquid condenser entering temperature;the liquid evaporator exiting temperature being less than the liquidcondenser entering temperature by a predetermined amount.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an exemplary embodiment for a heating, ventilation and airconditioning system.

FIG. 2 shows an isometric view of an exemplary vapor compression system.

FIG. 3 schematically illustrates an exemplary embodiment of the vaporcompression system.

FIG. 4 schematically illustrates a prior art embodiment taken fromregion 4 of the vapor compression system of FIG. 3.

FIG. 5 schematically illustrates a prior art embodiment taken fromregion 4 of the vapor compression system of FIG. 3.

FIG. 6 graphically illustrates a range of entering condensertemperatures and leaving evaporator temperatures of an exemplary vaporcompression system.

FIG. 7 graphically illustrates energy costs over a range of loadcapacity percentages versus a range of entering condenser temperaturesof an exemplary vapor compression system.

FIG. 8 graphically illustrates energy costs over a range of loadcapacity percentages versus a range of entering condenser temperaturesof an exemplary vapor compression system.

FIG. 9 graphically illustrates a range of load capacity percentagesversus a range of approach temperatures of the exemplary vaporcompression system.

FIG. 10 graphically illustrates a pressure-enthalpy curve for anexemplary refrigerant corresponding to a process cycle in an exemplaryvapor compression system.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

FIG. 1 shows an exemplary environment for a heating, ventilation and airconditioning (HVAC) system 10 incorporating a chilled liquid system in abuilding 12 for a typical commercial setting. System 10 can include avapor compression system 14 that can supply a chilled liquid which maybe used to cool building 12. System 10 can include a boiler 16 to supplyheated liquid that may be used to heat building 12, and an airdistribution system which circulates air through building 12. The airdistribution system can also include an air return duct 18, an airsupply duct 20 and an air handler 22. Air handler 22 can include a heatexchanger that is connected to boiler 16 and vapor compression system 14by conduits 24. The heat exchanger in air handler 22 may receive eitherheated liquid from boiler 16 or chilled liquid from vapor compressionsystem 14, depending on the mode of operation of system 10. System 10 isshown with a separate air handler on each floor of building 12, but itis appreciated that the components may be shared between or amongfloors.

FIGS. 2 and 3 show an exemplary vapor compression system 14 that can beused in an HVAC system, such as HVAC system 10. Vapor compression system14 can circulate a refrigerant through a compressor 32 driven by a motor50, a condenser 34, expansion device(s) 36, and a liquid chiller orevaporator 38. Vapor compression system 14 can also include a controlpanel 40 that can include an analog to digital (A/D) converter 42, amicroprocessor 44, a non-volatile memory 46, and an interface board 48.Some examples of fluids that may be used as refrigerants in vaporcompression system 14 are hydrofluorocarbon (HFC) based refrigerants,for example, R-410A, R-407, R-134a, hydrofluoro olefin (HFO), “natural”refrigerants like ammonia (NH3), R-717, carbon dioxide (CO2), R-744, orhydrocarbon based refrigerants, water vapor or any other suitable typeof refrigerant. In an exemplary embodiment, vapor compression system 14may use one or more of each of VSDs 52, motors 50, compressors 32,condensers 34 and/or evaporators 38.

Motor 50 used with compressor 32 can be powered by a variable speeddrive (VSD) 52 or can be powered directly from an alternating current(AC) or direct current (DC) power source. VSD 52, if used, receives ACpower having a particular fixed line voltage and fixed line frequencyfrom the AC power source and provides power having a variable voltageand frequency to motor 50. Motor 50 can include any type of electricmotor that can be powered by a VSD or directly from an AC or DC powersource. For example, motor 50 can be a switched reluctance motor, aninduction motor, an electronically commutated permanent magnet motor orany other suitable motor type. In an alternate exemplary embodiment,other drive mechanisms such as steam or gas turbines or engines andassociated components can be used to drive compressor 32.

Compressor 32 compresses a refrigerant vapor and delivers the vapor tocondenser 34 through a discharge line. Compressor 32 can be acentrifugal compressor, screw compressor, reciprocating compressor,rotary compressor, swing link compressor, scroll compressor, turbinecompressor, or any other suitable compressor. Compressor 32, as well asother rotating components of the vapor compression system, can includemagnetic bearings for providing smooth rotational movement. Therefrigerant vapor delivered by compressor 32 to condenser 34 transfersheat to a fluid, for example, water or air. The refrigerant vaporcondenses to a refrigerant liquid in condenser 34 as a result of theheat transfer with the fluid. The liquid refrigerant from condenser 34flows through expansion device 36 to evaporator 38. In the exemplaryembodiment shown in FIG. 3, condenser 34 is water cooled and includes atube bundle 54 connected to a cooling tower 56.

The liquid refrigerant delivered to evaporator 38 absorbs heat fromanother fluid, which may or may not be the same type of fluid used forcondenser 34, and undergoes a phase change to a refrigerant vapor. Inthe exemplary embodiment shown in FIG. 3, evaporator 38 includes a tubebundle having a supply line 60S and a return line 60R connected to aload or cooling load 62. A process fluid, for example, water, ethyleneglycol, propylene glycol, calcium chloride brine, sodium chloride brine,or any other suitable liquid, enters evaporator 38 via return line 60Rand exits evaporator 38 via supply line 60S. Evaporator 38 chills thetemperature of the process fluid in the tubes. The tube bundle inevaporator 38 can include a plurality of tubes and a plurality of tubebundles. The vapor refrigerant exits evaporator 38 and returns tocompressor 32 by a suction line to complete the cycle.

FIG. 4, which is taken from region 4 of FIG. 3, shows a prior artarrangement of components of a conventional vapor compression systemthat is configured for shut-down of a centrifugal compressor 32 (FIG. 3)during free cooling conditions, as previously discussed. As previouslydisclosed in the ASHRAE Handbook, only conventional vapor compressionsystems using centrifugal compressors were identified for use duringfree cooling conditions. As further shown in FIG. 4, a valve 26 ispositioned in fluid communication with each of the pair of linesextending between condenser 34 and cooling tower 56. Similarly, a valve26 is positioned in fluid communication with each of return line 60R andsupply line 60S extending between evaporator 38 and load or cooling load62. In response to a free cooling condition, each of valves 26 areclosed, preventing the flow of fluid from cooling tower 56 to condenser34 and the flow of fluid from load or cooling load 62 to evaporator 38.As a result, a heat exchanger 28, sometimes referred to as a water sideeconomizer, originally positioned between condenser 34 and cooling tower56, is now positioned in fluid communication with a closed cooling towerloop 30 that is in thermal communication with water from a closedcooling load loop 66. The water in closed cooling load loop 66 is movedby a pump 58 that is in fluid communication with closed cooling loadloop 66. During free cooling conditions, a valve 27 is opened, such thatpump 58 is in fluid communication with closed cooling load loop 66.

FIG. 5, which is taken from region 5 of FIG. 3, shows refrigerant flowof a prior art conventional vapor compression system that is configuredfor shut-down of centrifugal compressor 32 (FIG. 3) during free coolingconditions, as previously discussed, except with the difference that inFIG. 4, valve 27 is closed and valves 26 remain open. Upon shut-down ofcompressor 32 (FIG. 3), refrigerant in evaporator 38 has a naturaltendency to migrate to condenser 34, which typically operates at atemperature that is less than the temperature of evaporator 38. Onceliquid refrigerant migration from evaporator 38 to condenser 34 hasoccurred, and the pressures between condenser 34 and evaporator 38 haveequalized, by virtue of condenser 34 being positioned vertically aboveevaporator 38, liquid refrigerant begins to flow in a flow direction 68along line 70 toward evaporator 38, as a result of thermo siphoning.Upon the liquid refrigerant reaching evaporator 38, due to thetemperature in an evaporator 38 being greater than the temperature incondenser 34, an amount of liquid refrigerant in evaporator 38 “boils”or is changed to vapor refrigerant that moves in flow direction 72 thatis removed from evaporator 38. Once vapor refrigerant is removed,additional liquid refrigerant is then drawn into evaporator 38 as aresult of thermo siphoning, and the process is repeated. While thermosiphoning results in movement of an amount of refrigerant through theevaporator without operating the centrifugal compressor, the flow rateof refrigerant through the evaporator is significantly less whencompared with the flow rate that would normally occur during compressoroperation, thereby limiting the amount of cooling capacity available tosatisfy the demand for cooling. For example, as will be discussed infurther detail below, a conventional centrifugal liquid chiller, whenoperating in a free cooling condition or mode can generally onlyaccommodate approximately 12 percent of cooling load demand (% Load)when operating at an approach temperature of 3° F. (liquid evaporatorexiting temperature subtracted from the liquid condenser) (FIG. 9).

However, in an exemplary method of the present disclosure, vaporcompression system 14 (FIG. 3) operates differently than a conventionalcentrifugal liquid chiller during free cooling. That is, instead ofshutting-off a conventional centrifugal liquid chiller operating duringfree cooling conditions, the compressor of the exemplary vaporcompressor system of the present disclosure continues to operate at allenvironmental conditions (that is, environmental conditions which aresafe to operate a vapor compression system).

For example, in one exemplary method of operating a chiller having acompressor, while comparing the temperature of a liquid entering acondenser (“for thermal communication with refrigerant in thecondenser”) with a temperature of a liquid exiting an evaporator (forthermal communication with refrigerant in the evaporator), thecompressor is continuously operating at least in response to at leasteach temperature range identified herein. These temperature rangescomprise the liquid evaporator exiting temperature being greater thanthe liquid condenser entering temperature by a predetermined amount,which difference in temperatures (liquid evaporator exiting temperaturesubtracted from the liquid condenser) sometimes referred to as anapproach temperature. In one embodiment, the predetermined amount(approach temperature) is about 3° F. In another embodiment, thepredetermined amount (approach temperature) is greater than 3° F. Inanother embodiment, the predetermined amount (approach temperature) isbetween about 3° F. and about 5° F. For example, a user may determinethat the temperature difference be up to about 5° F. in one embodiment,or greater than 5° F., such as between about 5° F. and about 10° F. inanother embodiment. In yet another embodiment, the user may determinethat the temperature difference be greater than 10° F. FIG. 6 shows atemperature difference 74 (cross-hatched region) in which the leavingevaporator temperature is greater than the entering condensertemperature in amounts ranging from zero to about 15° F. It is to beunderstood by those skilled in the art that as the approach temperatureincreases, the amount of cooling load demand (% Load) that can beaccommodated by the chiller increases (see FIG. 9).

These temperature ranges also comprise the liquid evaporator exitingtemperature being substantially equal to the liquid condenser enteringtemperature. These temperature ranges also comprise the liquidevaporator exiting temperature being less than the liquid condenserentering temperature by a predetermined amount. For example, in certainapplications and/or temperature ranges of entering condensertemperatures and leaving evaporator temperatures, the user may select anincreased temperature difference, if the amount of cooling load demand(sometimes referred to as % Load) is generally low enough to beaccommodated by the chiller. The chiller cooling capacity is decreasedin response to an increased difference between the liquid evaporatorexiting temperature and the liquid condenser entering temperature, whenthe liquid evaporator exiting temperature is less than the liquidcondenser entering temperature.

These temperature ranges also comprise the liquid evaporator exitingtemperature fluctuating in response to a change in demand for chillercooling.

There are several advantages to continuously operating the compressor inresponse to all environmental conditions (within which are safe tooperate a vapor compression system). First, as previously discussed, thetemperature range (and accordingly, the probability of favorableenvironmental conditions) at which increased chiller operatingefficiencies are available is significantly increased. Second, whileproviding continuous compressor operation requires power, such aselectrical power, the amount of power required is minimized, due to theincreased chiller operating efficiencies associated with thesignificantly enlarged range of temperatures associated with favorableenvironmental conditions. For example, FIG. 7 shows a performance curve76 depicting a cooling capacity percentage (% Load) over a range ofentering condenser temperatures, and a performance curve 78 depictingthe energy required per unit of cooling (kW/Ton) of a liquid chillerover the same range of entering condenser temperatures.

In one embodiment, the compressor operates at or about a minimumrotational speed (“speed”), such as a minimal operating speed of VSD 52(FIG. 3) such as for rotatably driving compressor motor 50. In oneembodiment of a centrifugal compressor, a minimum operating speed of aVSD can be about 85 Hz. In other embodiments of centrifugal compressors,a minimum operating speed of a VSD may be significantly different than85 Hz. In addition, differences in minimum operating VSD speeds canresult because the vapor compression systems of the present disclosureare not limited to centrifugal compressors, but can include positivedisplacement compressors, including but not limited to reciprocating,rotary, swing link, scroll, and screw compressors, which may operate atdifferent minimum speeds. In one embodiment, for circumstances in whichoperating the compressor at a minimal speed provides an insufficientamount of cooling capacity to satisfy the demand for cooling, theoperating speed of the VSD would be increased, thereby increasing boththe compressor speed and cooling capacity until a sufficient amount ofcooling capacity is provided to satisfy the cooling demand. However, asshown in performance curve 80 of FIG. 8, if the chiller must operate atdesign cooling capacity (100% LOAD), the amount of energy per unit ofcooling (kW/Ton) increases with correspondingly increasing enteringcondenser water temperatures, such as with performance curve 80, whencompared to performance curve 78 (FIG. 7) at the same environmentalconditions and reduced cooling capacity.

In another embodiment, for circumstances in which operating thecompressor at a minimal speed provides an insufficient amount of coolingcapacity to satisfy the demand for cooling, the operating speed of theVSD would continue to operate at the same minimum speed. In other words,in this embodiment, the flow rate of water entering the evaporator andtemperature of water exiting the evaporator dictates the amount ofcooling available. As a result, an operator would be required toestablish operating constraints associated with favorable environmentalconditions associated with increased chiller operating efficiencies.Stated another way, the operator would need to control their system suchthat the water temperature exiting the evaporator, at a given flow rate,satisfies the cooling demand.

Third, by virtue of continued operation of the compressor of a chiller,a change in pressure is maintained in a suction line to the compressor32 (FIG. 3), thereby resulting in drawing liquid refrigerant fromcondenser 34 to evaporator 38, without thermal siphoning. In oneembodiment of evaporator 38, such as a falling film evaporator or hybridfalling film evaporator such as disclosed, for example in Applicant'sU.S. patent application Ser. No. 12/746,858 entitled “Heat Exchanger”,which is incorporated by reference in its entirety, liquid refrigerantin the evaporator cannot easily move between the condenser and theevaporator during shut-down of the compressor. As a result of the changein pressure due to continued compressor operation, which occurs even ata minimum compressor speed, liquid refrigerant is pushed from thecondenser and provided to the evaporator, permitting differentpositioning orientations of the evaporator relative to the condenser.Chillers needing to utilize thermal siphoning without a pump areconstrained to positioning the condenser vertically above theevaporator, as well as other plumbing constraints in order for thermalsiphoning to occur.

Fourth, as shown in FIG. 9, for a given approach temperature, whileoperating in a free cooling condition or mode (for purposes of providinga direct comparison with a conventional system constrained to operate ina free cooling condition), a liquid chiller continuously operating thecompressor at a minimum speed, for example 85 Hz., provides nearly twicethe design cooling capacity (% Load) when compared to a conventionalcentrifugal compressor utilizing thermal siphoning (e.g., FIG. 5). Alsoshown in FIG. 9 is a range of design cooling capacity (% Load) for atypical heat exchanger 28 (FIG. 4), sometimes referred to as a waterside economizer, operating with closed cooling tower loop 30 and closedcooling load loop 62 as previously discussed.

FIG. 10 shows a well-known pressure-enthalpy curve 82 representative ofdifferent phases (vapor, liquid and vapor, and liquid) of a refrigerantat different combinations of pressure and enthalpy of closed refrigerantloops ABCD and A′BCD′. (Although A, A′, B, C, D and D′ are subscripts ofenthalpy H, and normally expressed in a form such as HA, only thesubscripts by themselves are discussed below for purposes of clarity.)The closed refrigerant loop comprises a compressor 32, a condenser 34and an evaporator 38 (FIG. 3) and graphically depicts a process cycle(compression (BC), condensation (CD) and (C′D′), expansion (DA) and(D′A′), and evaporation (A′B) and (AB)) of the refrigerant duringoperation of the loop relative to the pressure-enthalpy curve of therefrigerant. Closed refrigerant loop ABCD corresponds to a process cyclein which the refrigerant is either a vapor or liquid and vapor. Closedrefrigerant loop A′BCD′ corresponds to a process cycle in which therefrigerant is one of a vapor, liquid and vapor, or a liquid.Cross-hatched region 84 corresponds to a segment of the process cycle inwhich the refrigerant is a liquid, such that operating the process cyclein the cross-hatched region 84 would result in more efficient heattransfer. An exemplary method of the present disclosure comprisescontinuously operating the compressor 32 when a segment of the processcycle corresponds to the refrigerant being in the liquid phase (i.e., asegment of cross-hatched region 84). As further shown in FIG. 10, asegment of cross-hatched region 84 can include at least a portion ofcondensation of the refrigerant occurring during the process cycle.Similarly, a segment of cross-hatched region 84 can include at least aportion of evaporation of the refrigerant occurring during the processcycle. Alternately, as further shown in FIG. 10, a segment ofcross-hatched region 84 can include at least a portion of each ofevaporation and of condensation of the refrigerant occurring during theprocess cycle.

While only certain features and embodiments of the invention have beenshown and described, many modifications and changes may occur to thoseskilled in the art (e.g., variations in sizes, dimensions, structures,shapes and proportions of the various elements, values of parameters(e.g., temperatures, pressures, etc.), mounting arrangements, use ofmaterials, colors, orientations, etc.) without materially departing fromthe novel teachings and advantages of the subject matter recited in theclaims. The order or sequence of any process or method steps may bevaried or re-sequenced according to alternative embodiments. It is,therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the true spiritof the invention. Furthermore, in an effort to provide a concisedescription of the exemplary embodiments, all features of an actualimplementation may not have been described (i.e., those unrelated to thepresently contemplated best mode of carrying out the invention, or thoseunrelated to enabling the claimed invention). It should be appreciatedthat in the development of any such actual implementation, as in anyengineering or design project, numerous implementation-specificdecisions may be made. Such a development effort might be complex andtime consuming, but would nevertheless be a routine undertaking ofdesign, fabrication, and manufacture for those of ordinary skill havingthe benefit of this disclosure, without undue experimentation.

What is claimed is:
 1. A method of operating a chiller having acompressor, the method comprising: comparing the temperature of a liquidentering a condenser (for thermal communication with refrigerant in thecondenser) with a temperature of a liquid exiting an evaporator (havingbeen in thermal communication with refrigerant in the evaporator);continuously operating the compressor at least in response to any of thefollowing conditions: wherein the liquid evaporator exiting temperatureis greater than the liquid condenser entering temperature by a firstpredetermined amount, wherein the liquid evaporator exiting temperatureis substantially equal to the liquid condenser entering temperature, andwherein the liquid evaporator exiting temperature is less than theliquid condenser entering temperature by a second predetermined amount;and operating the compressor at a minimum speed during free-coolingconditions wherein the liquid evaporator exiting temperature is greaterthan the liquid condenser entering temperature.
 2. The method of claim1, wherein continuously operating the compressor comprises continuouslyoperating the compressor in response to the liquid evaporator exitingtemperature fluctuating in response to a change in a demand for chillercooling.
 3. The method of claim 1, wherein the compressor is a positivedisplacement compressor.
 4. The method of claim 1, wherein thecompressor is a centrifugal compressor.
 5. The method of claim 1,wherein at least the compressor utilizes magnetic bearings.
 6. Themethod of claim 1, wherein the chiller comprises a VSD for controlling arotational speed of a compressor motor.
 7. The method of claim 1,wherein the first predetermined amount is between 1 degree Fahrenheit (°F.) and 15° F., and wherein the second predetermined amount is between1° F. and 40° F.
 8. A method of operating a chiller having a closedrefrigerant loop comprising a compressor, a condenser and an evaporator,the refrigerant used in the loop defining a pressure-enthalpy curverepresentative of different phases (vapor phase, liquid and vapor phase,and liquid phase) of the refrigerant at different combinations ofpressure and enthalpy, the loop defining a process cycle (a compressionsegment, a condensation segment, an expansion segment, and anevaporation segment) of the refrigerant during operation of the looprelative to the pressure-enthalpy curve of the refrigerant, the methodcomprising: continuously operating the compressor in response to a firsttemperature of a first cooling fluid leaving the evaporator beinggreater than a second temperature of a second cooling fluid entering thecondenser.
 9. The method of claim 8, wherein a portion of thecondensation segment of the process cycle corresponds to the refrigerantbeing a subcooled fluid in the liquid phase.
 10. The method of claim 8,wherein the compressor is a positive displacement compressor.
 11. Themethod of claim 8, wherein the compressor is a centrifugal compressor.12. The method of claim 8, wherein at least the compressor utilizesmagnetic bearings.
 13. The method of claim 8, comprising controlling arotational speed of a compressor motor via a VSD.
 14. The method ofclaim 8, wherein continuously operating the compressor includesoperating the compressor at least at a minimum speed.
 15. The method ofclaim 14, wherein continuously operating the compressor includesoperating the compressor at least at a minimum refrigerant flow ratesufficient to provide a required amount of cooling.
 16. A method ofoperating a chiller having a centrifugal compressor, the methodcomprising: comparing a temperature of a liquid entering a condenser(for thermal communication with refrigerant in the condenser) with atemperature of a liquid exiting an evaporator (having been in thermalcommunication with refrigerant in the evaporator); continuouslyoperating the compressor using a VSD for controlling a rotational speedof a compressor motor, the compressor utilizing magnetic bearings, atleast in response to any of the following conditions: wherein the liquidevaporator exiting temperature is greater than the liquid condenserentering temperature by a first predetermined amount, wherein the liquidevaporator exiting temperature is substantially equal to the liquidcondenser entering temperature, and wherein the liquid evaporatorexiting temperature is less than the liquid condenser enteringtemperature by a second predetermined amount; and operating thecompressor at a minimum speed during free-cooling conditions, whereinthe free-cooling conditions occur when the liquid evaporator exitingtemperature is greater than the liquid condenser entering temperature.17. The method of claim 16, wherein continuously operating thecompressor comprises continuously operating the compressor in responseto the liquid evaporator exiting temperature fluctuating in response toa change in a demand for chiller cooling.
 18. The method of claim 16,wherein continuously operating the compressor includes operating thecompressor at least at the minimum speed.
 19. The method of claim 16,wherein continuously operating the compressor includes operating thecompressor at least at a minimum refrigerant flow rate sufficient toprovide a required amount of cooling.